The present invention relates to a CRT display apparatus.
A display apparatus with a CRT is usually provided with various protection circuits for preventing burning of a fluorescent screen or an aperture grille in a case where an excessive electron beam flows towards the screen from an electron gun, or deflection or sweep of an electron beam is stopped under fault conditions, and thereby the beam concentrates at one point on the screen.
Generally, a muting circuit is used as such a protection circuit. There are various types of muting circuit, including the one that interrupts a video signal when an abnormal condition is detected, the one that shuts off the power for a video amplifier, the one that shuts off a high-tension of an anode, and the one that shuts off the power for a heater.
In the case of muting a video signal for protection, in order to interrupt the video signal when abnormality is detected, a muting circuit is disposed for each of three channels of R, G, and B at any of a receiving unit, a preamplifier, or a cathode-amplifier in a final stage within a video circuit.
On the other hand, the demand for improving resolution of CRT display apparatuses is growing in recent years. Japanese Unexamined Patent Publication No. 11-224618 discloses a high resolution CRT (referred to as xe2x80x9cHi-Gm tubexe2x80x9d hereinafter) that addresses such a demand.
An electron gun provided within a CRT has three electrodes of a cylinder form for drawing electrons from a cathode and prefocusing them, which are generally called xe2x80x9cG1 electrodexe2x80x9d, xe2x80x9cG2 electrodexe2x80x9d, and xe2x80x9cG3 electrodexe2x80x9d respectively, whereas an electron gun provided within the above-described Hi-Gm tube has, in addition to the G1, G2 and G3 electrodes, an electrode called xe2x80x9cGm electrodexe2x80x9d disposed between the G2 electrode and the G3 electrode for modulating an electron beam.
FIG. 5 shows a structure of such an electron gun used for the Hi-Gm tube. In this drawing, 17 denotes a G1 electrode, 18 denotes a G2 electrode, 20 denotes a cathode, 21 denotes an electron-emitting substance formed on the surface of the cathode 20, and 22 denotes a Gm electrode. The electron gun has, for the part following the G3 electrode in which other focusing electrodes are disposed, the same structure as the conventional electron gun.
FIG. 6 is a graph showing potential distribution near the cathode of the electron gun within the Hi-Gm tube. In this graph, the horizontal axis represents the distance (mm) from the cathode surface, the vertical axis represents the potential (V), and the curve marked by 23 shows the potential distribution symmetrical with the axis of revolution near the cathode. Furthermore, the arrow marked by 24 shows the range within which the Gm electrode 22 exists, which is about 0.5 mm from the cathode surface.
The potential of the Gm electrode 22 is set to about 80 VDC, so there is a position 25 within the range at which the level of the spatial potential is minimum. If the potential of the cathode 20 shown by the dashed line is lower than the potential at this position 25, electrons pass through the position 25 and flow towards a screen. If not, electrons do not flow towards the screen since they cannot pass through the position 25.
As seen from the graph of FIG. 6, between the cathode and the position 25, electrons always exist abundantly, and the slope of the potential behind the Gm electrode 22 is of the order of 106 (V/m). Compared with the potential slope between the cathode and the G1 electrode, it is greater by an order of magnitude. Therefore, after electrons pass through the Gm electrode 22, most of them can move towards the screen without being affected by spatial charges, so the intensity of the electron beam flowing to the screen is determined by the quantity of the electrons that pass through the position 25 where the spatial potential is minimum.
For this reason, variation of the intensity of the electron beam when the cathode potential is varied by a certain value in the Hi-Gm tube is about twice as much as that in the conventional CRT. That is, the variation of the cathode potential required to vary the intensity of the electron beam by a certain value in the Hi-Gm tube is less than half the variation required in the conventional CRT. In other words, with the Hi-Gm tube, the variation of the intensity of the electron beam can be doubled for the same variation of the cathode potential. Consequently, with the Hi-Gm tube it is possible to easily adapt to video signals of high frequency.
However, in the above-described Hi-Gm tube, an electron beam flowing to the screen when the cathode voltage falls abnormally in the event of failure will be much greater, and a spot beam which occurs if deflection or sweep of an electron beam is stopped abnormally will be much greater compared with those in the conventional CRT. Accordingly, a spot burning of a fluorescent screen or a burning of an aperture grille can occur more easily than ever before.
In the case of carrying out the protection by muting a video signal, a muting circuit is provided for each of R, G, and B channels individually. However, since there is variation in operation timings and signal attenuation levels among such muting circuits, it is not necessarily possible to obtain desired protection by such muting circuits in the case of using the Hi-Gm tube.
An object of the present invention is to prevent occurrence of a spot burning of a fluorescent screen and a burning of an aperture grille of the Hi-Gm tube provided with an electron gun having a Gm electrode.
This object is achieved by a CRT display apparatus including a CRT having an electron gun; the electron gun including:
a cathode;
a G1 electrode, a G2 electrode, and a G3 electrode disposed in that order for drawing electrons from the cathode; and
a modulating electrode disposed between the G2 electrode and the G3 electrode;
wherein the CRT display apparatus is provided with a controller for controlling a value of a voltage applied to the modulating electrode in order to control intensity of an electron beam flowing from the cathode to a screen of the CRT.
The controller may include:
a generator generating a protection signal when at least one of an excessive electron beam, an overvoltage of an anode of the CRT and a stoppage of deflection of the electron beam is detected; and
a voltage source which, upon receiving the protection signal, alters a value of an output voltage of the voltage source being applied to the modulating electrode in order to suppress or interrupt the electron beam flowing from the cathode to the screen.
The controller may include a voltage source which is powered by a power supply of a deflection circuit for generating a signal used for deflecting the electron beam flowing from the cathode to the screen, and generates the voltage applied to the modulating electrode.
The controller may include a voltage source which is powered by a power supply of a video circuit for applying a voltage according to a video signal to said cathode, and generates the voltage applied to the modulating electrode.
The controller may include a voltage source which, upon receiving an image-muting signal from outside, alters a value of its output voltage being applied to the modulating electrode in order to blank out the screen.